This invention relates generally to frequency shifters and particularly to fiber optic frequency shifters. Still more particularly, this invention relates to a fiber optic frequency shifter for use in an optical rotation sensing system to shift the frequency of light input to a frequency suitable for the angular rotation rate to be detected.
A fiber optic ring interferometer typically comprises a loop of fiber optic material having counter-propagating light waves therein. After traversing the loop, the counter-propagating waves are combined so that they constructively or destructively interfere to form an optical output signal. The intensity of the optical output signal varies as a function of the type and amount of interference, which is dependent upon the relative phase of the counter-propagating waves.
Fiber optic ring interferometers have proven to be particularly useful for rotation sensing. Rotation of the loop creates a relative phase difference between the counter-propagating waves, in accordance with the well known "Sagnac" effect, with the amount of phase difference being a function of the angular velocity of the loop. The optical output signal produced by the interference of the counter-propagating waves varies in intensity as a function of the rotation rate of the loop. Rotation sensing is accomplished by detecting the optical output signal and processing the optical output signal to determine the rotation rate.
In order to be suitable for inertial navigation applications, a rotation sensor must have a very wide dynamic range. The rotation sensor must be capable of detecting rotation rates as low as 0.01 degrees per hour and as high as 1,000 degrees per second. The ratio of the upper limit to be measured and the lower limit is approximately 10.sup.9.
The output of an open loop fiber optic gyroscope is a sinusoidal waveform. The sinusoid is nonlinear and not single valued, which presents difficulties in obtaining accurate measurements. The amplitude also fluctuates because it is dependent upon several parameters that may fluctuate.
The principle of using a bulk optics rotating half-wave plate as a frequency shifter is well known, and such rotating half-wave plate frequency shifters are used at both microwave and optical frequencies. Optical frequency shifting may be accomplished by passing an optical signal through an electro-optic crystal having a three-fold axis and applying a rotating electric field to the crystal. To be frequency shifted, the optical beam is preferably circularly polarized and directed along the three-fold axis of the crystal. With no field applied, the crystal exhibits no birefringence, and the emergent beam is unaffected. When the applied field has the proper half-wave amplitude and rotates in a plane normal to the three-fold axis, the crystal functions as a rotating half-wave plate. The emergent beam has its optical frequency shifted and its sense of polarity reversed. The frequency shift is equal to the rotation rate of the applied field. Because a uniformly rotating applied field ideally results in a single new frequency in the output beam, rotating field frequency shifters are referred to as single-side-band-suppressed carrier (SSBSC) modulators.
Previous frequency shifters employ nonlinear interactions between acoustic and optical waves and suffer from small band widths, difficult geometries and the requirement for special optical fibers and are unable to provide the accuracy required for airplane navigation.